Subterranean formations of oil and gas wells are often treated by hydraulically fracturing the formations to increase the production of oil or gas. Fracturing of the formations is accomplished by pumping fluids into the bore holes of the oil or gas wells under high pressure so that cracks or fissures are opened into the surrounding formation. Typically, the fracturing fluid contains a polymer which will gel the fluid to increase its viscosity. The fluid's viscosity is proportionally related to the created fracture geometry and fracture width so that the more viscous fluids will produce longer and wider fractures. After the fracturing fluid is injected into the formation to produce the fracture, the viscosity of the fluid is reduced by means of “gel breakers” which break down the gelled fluid so that it can be easily pumped and removed from the well.
In certain formations, aqueous acid solutions are used to improve the permeability of the formation, thereby increasing production. These acids are often combined with the polymer gels used in fracturing to provide an acid fracturing fluid. Besides the normal benefits of increasing the efficiency of the fluid's fracturing capabilities (leak-off control, etc.), the polymer gelling agent also provides a barrier to slow down the acid reaction. This is beneficial in that the acid would otherwise react too quickly, depleting the acid with very little penetration of the formation. Once in place, the viscosity of the fluid is reduced so that the acid is released to react with formation damage or other skin present at the face of the newly formed fractures and improving the permeability of the producing strata.
Crosslinked polymer gels have been found to be useful with these aqueous acid solutions. Crosslinked gels are able to withstand the high temperature conditions commonly found in deeper oil and gas wells with little reduction in viscosity, and they exhibit an improved ability in reducing the reaction rate of the acid solution. Organometallic compounds are often used as a crosslinking agent in these polymer gels. Unfortunately, crosslinked acid systems using synthetic polymers and metal crosslinking agents have little to no delay due to the reactiveness of the polymer and metal in acid systems. Typical chelating agents have had little effect on delaying these systems especially where the systems are premixed for 12 or more hours, which often are the case during acid treatments.
A small amount of lactic acid or salts thereof have previously been used in water based fracturing fluids containing sand and a non-synthetic polymer, such as guar or cellulose or cellulosic derivatives, to delay crosslinking. However, such systems exhibit poor stability in aqueous acid solutions.